The present invention pertains to a mechanical positioning system and more particularly, to a positioning system for an ultrasonic therapy device operating under Magnetic Resonance Imaging guidance.
The use of Magnetic Resonance Imaging (MRI) is well-known. MRI provides a radiologist with detailed internal views of a patient""s anatomy that cannot be visualized with conventional x-ray technology. Images generated by MRI systems provide physicians with a visual contrast between varying tissues that is extremely useful when planning surgical procedures.
Ultrasonic therapy uses focused, localized heating to selectively destroy tumors or other tissue anomalies. Heating tissue beyond a critical temperature for a period of time causes necrosis, the destruction of tissue. The use of MRI imaging to guide the focal point of an ultrasonic therapy device is known. For instance, U.S. Pat. Nos. 5,443,068, 5,275,165, and 5,247,935 each describe the use of an ultrasonic transducer, guided by an MRI system, to selectively destroy tissue. The details of these disclosures are hereby incorporated by reference into the present application.
The need to accurately position an ultrasonic transducer for use in selective tissue necrosis presents special problems when used in combination with an MRI guidance system. In particular, MRI systems employ large magnets, for creating a homogenous magnetic field, and gradient coils for altering that magnetic field in a uniform manner in time and/or space. This procedure creates magnetic field gradients. MRI systems also employ radiofrequency (RF) coils for applying an RF field to the tissue that is to be imaged, causing the tissue to resonate and create an MR response signal. The MR response signal is then used to construct an image of the tissue that is displayed to an operator. The image can then be printed or otherwise stored for later use and analysis. The degree of homogeneity of the magnetic field and the linearity of a magnetic field gradient over space and time are important in creating a clear undistorted image. Any interference with the RF field will reduce the quality of the image. The best and most consistent imaging typically occurs when surgical equipment or other objects do not interfere with the magnetic and RF fields created by the MRI system.
Several situations may affect the performance of MRI systems or other equipment used in conjunction with it. For example, equipment that is constructed from ferro-magnetic materials should not be used near an MRI system since the large magnetic fields generated by the MRI system will physically attract the magnetic equipment. Consequently, MRI performance may suffer. Furthermore, conductive materials disturb and distort the radio frequency electromagnetic fields necessary for resonance imaging. Other problems occur with materials that produce eddy currents when placed in a time-varying magnetic field. The eddy currents in these materials, usually electrical conductors, create their own magnetic field that interferes with the fields used for magnetic resonance imaging. Therefore, materials which exhibit good conductivity, such as aluminum and copper, should not be used within a time-varying magnetic field.
In order to accurately position an ultrasonic therapy device under MRI guidance, a precise positioning system must be employed. In addition, the positioning system needs to be able to provide repeatedly predictable control of the ultrasonic transducer in order to accommodate the precision requirements of certain clinical procedures. Tumors that may be small or have irregular shapes require exact positioning of the ultrasonic transducer in order to destroy only the intended tissue while leaving the healthy tissue undamaged.
Known positioning systems, such as those described in U.S. Pat. Nos. 5,247,935 and 5,275,165 utilize hydraulic mechanisms to position an ultrasonic transducer beneath a patient. However, these systems rely on placing the transducer directly beneath the object to be treated (e.g., a tumor) and provide positioning only in the linear x, y, and z axes. Due to constraints that may be imposed by the available acoustic passage to the object to be treated, effective therapy may not be possible with this arrangement. Additionally, these known systems have inherent reliability and accuracy problems due to the use of hydraulic positioners which can exacerbate motor backlash, degrading the accuracy of the positioner.
Since the motors used in these known systems are formed from materials that interfere with the operation of the MRI system, the motor must be placed at an increased distance from the ultrasonic transducer and the MRI imaging space. Known positioning systems therefore require the use of long motor drive shafts, which increase the physical footprint of the positioning system. Furthermore, the motors used in known systems need to be left engaged and energized in order to minimize slippage due to the backlash problem. Since an energized motor produces an increased electric field that interferes with the operation of a MRI system, the motor cannot be mounted within, or in the vicinity of, the MRI imaging space. For this reason, known systems require the motors to be mounted at a significant distance from the MRI imaging space.
U.S. Pat. No. 5,443,068 describes an MRI guided ultrasonic therapy system that uses threaded shafts attached to screw drives through universal joints in order to position the therapy transducer in three linear dimensions. The ""068 patent also requires the ultrasonic transducer to be placed directly beneath the object to be treated and only provides positioning in the linear x, y, and z axes. The screw drives, and particularly the universal joints, utilized in this system compound the motor backlash problem described above and therefore further restrict the positional accuracy that the system can achieve. Furthermore, the motor drives of the ""068 patent are formed from magnetic material and must also be located at a distance from the imaging space in order to eliminate interference with the MRI system.
The foregoing problems are solved by providing a positioning system for magnetic resonance imaging guidance of a therapy device. In a first embodiment, the position system comprises an energy transducer, a first positioner operative to adjust the location of the energy transducer in a lateral direction in a first plane, a second positioner operative to adjust the location of the energy transducer in a longitudinal direction in the first plane, and a third positioner operative to adjust the roll of the energy transducer.
Each of the positioners are vibrational motors that comprise a drive shaft and a linear actuator coupled to the drive shaft. The actuators are operative to produce a rotary motion of the drive shaft. The positioning system may further comprise a fourth positioner operative to adjust the pitch of the drive shaft.
In a further embodiment a device for positioning an ultrasonic therapy device under magnetic resonance imaging comprises a motor coupled to a drive shaft, a first position encoder coupled to the motor, and a second position encoder coupled to the drive shaft. The first position encoder is operative to measure the amount of motion relative to a predetermined position and the second position encoder is operative to measure the amount of motion relative to an arbitrary position.
In another embodiment, a device for positioning an ultrasonic therapy device under magnetic resonance imaging guidance comprises a vibrational motor coupled to a drive shaft, and a positioner assembly that includes a support bracket, a longitudinal slide coupled to the support bracket, and a lateral slide coupled to the longitudinal slide. The device may further comprise a second vibrational motor coupled to a second drive shaft, a third vibrational motor coupled to a third drive shaft, and a fourth vibrational motor coupled to a fourth drive shaft. Each of the vibrational motors controls a specific directional motion of the support bracket.
In a still further embodiment, a device for positioning an ultrasonic therapy device under magnetic resonance imaging guidance comprises energy concentrating means for directing energy at a focal point, a first positioning means for adjusting the lateral position of the energy concentrating means, a second positioning means for adjusting the longitudinal position of the energy concentrating means, and a third positioning means for adjusting the roll of the energy concentrating means. The device may further comprise a fourth positioning means for adjusting the pitch of the energy concentrating means.
Other and further aspects and advantages of the invention will become apparent hereinafter.